This invention relates to tuning control apparatus for a frequency synthesizer tuner and, more particularly, to such tuning control apparatus wherein the tuning condition of the frequency synthesizer tuner can be changed at a variable rate or, alternatively, the tuning condition can be changed in an incremental step-wise manner under the control of an operator.
In a typical tuner, such as a tuner for receiving broadcasted radio or television signals which are transmitted over respective broadcast frequencies, the tuning condition of the tuner is determined by the frequency of the local oscillating signal which is mixed with the received broadcast frequencies to produce an intermediate frequency (IF) signal. The carrier frequency of the IF signal is constrained within a narrow range which is a function of the mixing of the broadcast frequency and the local oscillating frequency. As the local oscillating frequency is changed, the tuner is tuned to different broadcast frequencies to receive the program information which is broadcasted thereover. Typically, a local oscillator may include a manually adjustable capacitor which, when the capacitance value thereof varies, the local oscillating frequency correspondingly varies. By adjusting the tuning knob which is mechanically coupled to the variable capacitor, an operator may change the local oscillating frequency as desired and, thus, may establish any desired tuning condition of the tuner. Recently, the variable capacitor has been constructed as a variable capacitance diode whose capacitance value is determined by a control voltage applied thereto. Since the same control voltage will result in the same tuning condition, tuners are known wherein digital techniques are relied upon for storing digital representations of respective control voltages, which digital representations can be retrieved, as desired, so as to establish a capacitance value rapidly which would tune the tuner to a desired broadcast frequency.
More recently, a so-called frequency synthesizer tuner has been proposed, in which the local oscillating signal is generated by a phase-locked loop under the control of a digital frequency-selecting signal. In such a phase-locked loop, a variable frequency oscillator produces the local oscillating signal. In addition to being supplied to the usual mixer in the tuner, the local oscillating signal is supplied through a programmable frequency divider to a phase comparator whereat it is compared to a reference oscillating signal. Any phase difference therebetween results in an error signal which is fed back to the variable frequency oscillator so as to adjust the frequency of the local oscillating signal and thereby adjust the tuning condition of the tuner. If the dividing ratio of the frequency divider changes, the frequency-divided oscillating signal which is supplied to the phase comparator will change. By well-known phase-locked loop operation, this changes the basic frequency of the local oscillating signal, resulting in cancelling the phase error signal. Thus, the tuning condition of the tuner is established merely by setting a desired frequency dividing ratio of the programmable frequency divider.
In the aforementioned frequency synthesizer tuner, the dividing ratio of the programmable frequency divider may be established by a counter, such as an UP/DOWN counter whose count sets the dividing ratio. As the count of this UP/DOWN counter is incremented, the dividing ratio increases and, conversely, as the count of the UP/DOWN counter is decremented, the dividing ratio correspondingly decreases. This features can be used, advantageously, to effect a so-called scanning operation, whereby the tuning condition of the tuner is scanned either in the upward or downward direction from one broadcast frequency to the next. Such a scanning operation may be helpful to the operator to enable him to ascertain the program information which is available on the various broadcast frequencies which can be received by his tuner. This scanning operation may be of the automatic scanning type, referred to herein as the auto-scan mode, in which the count of the UP/DOWN counter is incremented or decremented periodically at a fixed rate, whereupon the frequency of the local oscillating signal is increased or decreased at this same rate. One disadvantage of such an auto-scan mode is that the rate at which the tuning condition of the tuner is changed is fixed. In many instances, this fixed rate may be too slow for the user. However, heretofore, it has not been possible for the user to change the auto-scan rate.
In addition to the auto-scan mode, it may be desirable to change the tuning condition of the tuner on a step-wise basis. That is, although the auto-scan rate may, in many instances, be too slow, in other instances the auto-scan rate may be too fast. In view of the latter disadvantage, it is desirable to change the tuning condition by an incremental amount (for example, by 0.1 MHz for an FM tuner and by 1 KHz for an AM tuner) in response to each manual operation by the user.